An ultrasonic diagnostic device is a device which takes a diagnostic image based on a reflection echo signal output from an ultrasonic probe. A plurality of ultrasonic transducers are arranged at the ultrasonic probe. The ultrasonic transducer converts a driving signal into an ultrasonic wave, transmits ultrasonic wave to a subject, receives the reflection echo signal generated from the subject and converts it to an electric signal.
Recently, an ultrasonic probe that uses a cMUT (Capacitive Micromachined Ultrasonic Transducer) has been developed. The cMUT is a super-minute capacity type ultrasonic wave transducer manufactured using a semiconductor microfabrication process. In the cMUT, an ultrasonic wave transmitting/receiving sensitivity, in other words, electromechanical coupling coefficient changes depending on the magnitude of a bias voltage. In addition, the bias voltage is superimposed on the driving signal provided by an ultrasonic wave transmitting/receiving part and is applied (For example, refer to patent document 1).    Patent Document 1: U.S. Pat. No. 5,894,452